The reaction of the pro-ligand H4L, which combines the complementary phenolic oxime and diethanolamine moieties within the same organic framework, with Zn(NO3)2·6H2O and Ln(NO3)3·6H2O in a basic methanolic solution generates a family of isostructural heterometallic coordination compounds of general formula [Ln6Zn2(CO3)5(OH)(H2L)4(H3L)2(H4L)]NO3·xMeOH [Ln = Gd, x = 30 (1), Ln = Dy, x = 32 (2), Ln = Sm, x = 31 (3), Ln = Eu, x = 29 (4), Ln = Tb, x = 30 (5)]. The octametallic skeleton of the cage describes a heavily distorted [Gdlll6] octahedron capped on two faces by ZnII ions. The metal core is stabilised by a series of μ3- and μ4-CO32- ions, originating from the serendipitous fixation of atmospheric CO2. The magnetic properties of all family members were examined via SQUID magnetometry, with the χMT product and VTVB data of the Gd analogue (1) being independently fitted by numerical diagonalisation to afford the same best-fit parameter JGd-Gd = -0.004 cm-1. The MCE of complex 1 was elucidated from specific heat data, with the magnetic entropy change reaching a value of 22.6 J kg-1 K-1 at T = 1.7 K, close to the maximum entropy value per mole expected from six GdIII spins (SGd = 7/2), 23.7 J kg-1 K-1.